AO39 conversion to Sluckey Vox AC15 Lite Troubleshoot.

[Ronno25]

Hi all -

I've been reading this forum for sometime without posting. This is my first post. A friend and I were keen on building the Sluckey Vox AC15 lite from AO39 Hammond chassis and transformers. She built hers from the schematic and photo that Sluckey shared on his website (thank you Sluckey!). It sounds great, and most importantly it works! Which brings me to my next point.

I built the identical amp from the same parts and having been having problems. When I built it I had my friend's right in front of me and pretty much copied it visually. But I conferred with Sluckey's schematic as well. My friend has also gone over it visually and found no issues.

The problem: The amp is blowing fuses when plugged straight into the wall. When used with a 150watt lightbulb current limiter and all tubes installed the current limiter does no glow too bright and I can play the amp and it sounds good (taking into account the influence of the current limiter on the sound). With the current limiter in use the voltages also appear correct at somewhat lower than the numbers included on Sluckey's schematic. With no tubes installed and the current limiter in use the light bulb on the current limiter glows brightly.

What I've tried: I convinced myself that the PT had an internal short and wrangled up a new one of the same model. Installing the new PT made no difference. The problem appears identical. I also installed a new power cable to no difference.

I'm new to the amp building world and electronics. I've built two amps prior without any issues in the power section. That being said my electrical knowledge is limited (and so is my friend's). Any and all help would be super appreciated. I posted here with the idea people may be familiar with this build (and maybe Sluckey himself would chime in). This amp sounds so good when working and I'm really hoping to get it going. Thanks in advance for your help!

[shooter]

blowing fuses

what value fuse?  SB or FB?
what's the VDC at the 1st tap (typically the PA tube tap) with the limiter in?
does the fuse blow instantly or after a few power chords
is the limiter 2 prong or 3?

[Ronno25]

blowing fuses

what value fuse...

2A Slo Blo Fuse.

I'm not sure what PA tube is. But the VDC at the first filter cap with current limiter in is 192vdc. I had the probe connected before turning the amp on and noticed that when first switched on the VDC was over 300 initially but went down to the 192 after the rectifier warmed up.

The amp blows fuses instantly. The limiter is 3 prong. But I noticed the power strip it was plugged into has an intermittent ground. I did plug my friends identical amp into that same strip with no problems, however.

[Ronno25]

I'm getting 70vdc on pin 1 and 6 of the EF86 in V1 with current limiter in. If you meant preamp by PA.

[shooter]

a hundred volt drop is BIG, even bigger without the limiter

With no tubes installed and the current limiter in use the light bulb on the current limiter glows brightly.

You have a partial or full on short.

with power off, caps discharged;
take a gatorclip and short 1st cap + to ground (shorting out the cap)
connect your meter reading Ohms, ground lead to ground, unground the cap gatorclip at ground and connect to meter + (effectively cap +)


wait a couple minutes till everyone is happy and meter quits climbing - ish, what is the Resistance??


good is something very high Kohms to MegOhms

[sluckey]

What kind of bulb is in that lamp holder? Not a neon I hope.  Remove it and test amp without that bulb. Any better?

[Ronno25]

Full Voltage Chart w/ Current Limiter in Use:

V1 EF86:
p1 70
p2 0
p3 1.1
p4 2VAC to ground
p5 2VAC to ground
p6 77
p7 0
p8 1.1
p9 0

V2 12AX7:
p1 131
p2 22
p3 32
p4 2VAC to ground
p5 2VAC to ground
p6 136
p7 23
p8 33
p9 2VAC to ground

V3 EL84
p1 2.7mv
p2 2.7mv
p3 4.9
p4 2VAC to ground
p5 2VAC to ground
p6 .6mv
p7 186
p8 10mv
p9 181

V4 EL84
p1 4.6mv
p2 4.2mv
p3 4.9
p4 2VAC to ground
p5 2VAC to ground
p6 0
p7 186
p8 19mv
p9 179

V5 5y3
Across p2 and p8: 3.3VAC
Across p4 and p6: 400VAC

B+1 190
B+2 180
B+3 170
B+4 170

[Ronno25]

It's this one: https://www.tubesandmore.com/products/dial-lamp-ne51-t-3-14-90110v-0003a-bayonet-base

I think it is a neon. And I just saw it requires a current limiting resistor, which I didn't install.

Okay I just plugged the amp in without the current limiter with the bulb removed. No blown fuse.

[sluckey]

Yes, it requires a resistor. When you install the resistor fix that dangerous connection you have. Connect the wire from the white neutral wire to the side lug on the lamp holder. Remove the wire from the switch and discard. Now connect the resistor from the power switch to the end lug of the lamp holder. This will prevent you from getting a nasty shock when you remove the lamp bulb.

[Ronno25]

Thanks, Sluckey. I'll rewire the lamp and add a resistor.

The amp is working now and the voltages check out across the board. There are however some odd and unpleasant noises connected to the tone and volume pots. The amp sounds normal otherwise as far as I can tell with just a speaker on the table. The oddities are:

1. Both the volume and tone pots loudly pop when sliding the control at about 1/4 way up.

2. There is a loud whistle type sound that is constant that triggers at about 2/3 to full up on the control.

3. There is a very loud TV static sound that comes in around 2/3 up on the control and goes away if you raise or lower the pot a little bit.

Any ideas?

[sluckey]

Look for a poor connection. Be sure all your ground connections are secure. Use a chopstick to probe every connection, component, and wire.

Post another set of voltage readings without the current limiter. In fact, you no longer need the current limiter. Put it on a shelf.

[Ronno25]

Voltage Chart without current limiter:

V1 EF86
p1 89
p3 1.9
p7 100
p8 1.97

V2 12AX7
p1 194
p2 44
p3 64
p6 199
p7 44
p8 64

V3 EL84
p3 10
p7 295
p9 282

V4 EL84
p3 10
p7 295
p9 283


V5 5y3
Across p2 and p8: 5.32VAC
Across p4 and p6: 630 VAC

B+1 302
B+2 283
B+3 265
B+4 264.5

All heater filaments were 3.26 to ground.

I'll do some prodding with a chopstick and see what I can come up with. Thanks again!

[Ronno25]

My prodding with a chopstick didn't uncover any issues. However, I took a jumper cable to all the ground points to make sure they were grounded to chassis. What I discovered is that when I bypassed the 470k resistor that is in series with the 8k2 coming off of V4 pin2 all squealing, loud static noise, and most of the pop went away when adjusting the pots.

As a solution I tried resoldering all the joints in that part of the circuit, which did not fix the problem. So I'm wondering if replacing the 470k resistor with a lower value would be a solution, or removing that resistor altogether? I'm unfamiliar with if that would cause other problems. However, when I bypassed that resistor to ground I played through the amp and it sounded fine. I circled the resistor in question in green in the attached image.

Also worth mention is that I didn't follow the exact grounding scheme that Sluckey used as I noticed there were wires running between the different grounds that seemed redundant. Are these a potential source of my issues or is it just best practice to have all the grounds connected?

[sluckey]

The resistor is not likely to be the problem. Measure resistance from V4 pin 2 to chassis. What have you?

[Ronno25]

Measure resistance from V4 pin 2 to chassis. What have you?

.482M with tube installed.

[sluckey]

Measure the voltage on V4 pin 2. Do the same for V3.

[Ronno25]

V4p2 The meter is shifting between 7mv and 12mv DC.

V3p2 Shifting between 7mv and 11mv DC.

----
Other readings from power tubes that I didn't include in my initial voltage chart:

V4p1 between 7mv and 12mv DC.
V4p8 between 4mv and 16mv DC.
V3p2 between 7mv and 11mv DC.
V3p8 between 5mv and 20mv DC.

[sluckey]

If you set the VOL to zero, do all the noises go away? Does the CUT control still produce pops when turning?

[Ronno25]

Vol at zero all noises are gone except the pop remains when turning the tone pot.

Cut pot at zero the volume still makes pops but the other noises are gone.

[sluckey]

We need to get together on our terminology. There are only two pots in this amp... VOLUME and CUT. There is also a six position switch that could be called a tone switch. So, what are you calling the tone pot?

[Ronno25]

I was referring to Volume and Cut in my previous post. I’ll edit that post to make clear.

The six position rotary tone switch also makes pop sounds when switching between settings. This pop also went away when bypassing the 470k resistor to ground.

[Ronno25]

I swapped out all tubes one at a time with no change to aforementioned noise problems. Sluckey, any more ideas of what to try? I really appreciate your time on this one, thank you.

[sluckey]

You could hard wire all your ground points like the original AO-39. Hammond used a lot of rivets to fasten things to the chassis and they didn't trust the rivets to give a solid ground connection.

[Ronno25]

What do you mean by hard wire? Excuse my ignorance here. So your diagnosis of the problem is that it's a ground issue?

[sluckey]

 My grounding suggestions is not a diagnosis, rather like a wild ass guess (WAG). Look at the original AO-39 layout and you'll see that all the ground points are wired together.

I suggest you get together with your friend again and compare amps closely, but this time look for bad/loose connections, especially with any ground connections. And be sure all your chassis connections are secure.

[Ronno25]

Thanks, Sluckey. I've tried doing the grounding scheme as close as I can to what I see in the pick of your amp. Unfortunately, it didn't help with the noise problems.

I've linked a short video of the noise problems here in case anyone wants to give a listen:

[glass54]

Ronno
Very interesting soundclip 
It sounds to me as if its unstable (motorboating? somewhere above say 200kHz and max of say 3MHz)
Turning your knobs and getting some variation occasionally almost definitely gives it away.
But Why?? I built a prototype 2 tube preamp last year on veroboard (MISTAKE!!) and got a similar problem, esp with treble control.
So I scrapped it and built it on tagboard and it was absolutely perfect.
Do you have an Oscilloscope? Or access to one?
Kind regards
Mirek

[Ronno25]

Thanks so much for the response, Mirek. Why it's making these noises is beyond me. I have very limited electrical knowledge. Funny you mention rebuilding the amp. I've been making a layout of the schematic for the last couple hours. This amp sounds really awesome and it's definitely something I would use for recording if not live. So I'll likely rebuild the amp in a format that's more familiar to me using an eyelet board design. I'll keep this thread posted as I go.

A friend has an old oscilloscope but we both lack the know-how to use it . I could probably figure it out with enough time.

[sluckey]

Have you tried other tubes? Especially the EF86? I don't think I can troubleshoot this any farther without actually having my hands on the amp.

If you are considering rebuilding you may be interested in my Dual Lite. That's what I finally built with my AO-39. Very simple amp.

     http://sluckeyamps.com/dual_lite/dual_lite.htm

[Ronno25]

Cool build, Sluckey. As it happens I was planning to build an 18w Marshall after this build. So building that dual amp may be just what I need!

I already tried swapping all the tubes, including the EF86. Unfortunately, it didn't help.

Thanks for all your help on this one. I'll keep ya'll posted with what I go forward with. In the meantime I made a little recording to demo just how cool this AC15 Lite sounds. This is a strat on bridge pickup, Volume max, and Cut min. Could be helpful for future people wanting to build this amp.